Hollow silica opals/cellulose acetate nanocomposite films with structural colors for anti-counterfeiting of banknotes

Utilizing biomass materials to construct photonic bandgap materials has attracted increasing attention due to their environmental friendliness. In this work, we introduced biodegradable cellulose acetate to the interstice of hollow silica 3D photonic crystals (PCs) for building hollow silica opals/cellulose acetate nanocomposite films. In the films, the hollow silica 3D PCs offered well-ordered arrays for producing structural colors. Cellulose acetate with excellent film forming ability served as a matrix endowing the PCs frameworks with mechanical stability, including flexibility, robust tensile strength and the ability of repetitive bending. The prepared colored composite films possessed high transparency and can work as anti-counterfeiting labels for banknotes when patterned by a facile mask method. Compared with other reported PCs which are responsive to external stimuli in the field of anti-counterfeiting, these films have the advantage of quick color-transparency switching just by changing the viewing angle. Except for application in security materials, the colored composite films also showed potential as package materials due to their biodegradability.